1. Field of the Invention
The present invention relates to a new and improved method and apparatus for measuring conductive workpiece surface characteristics such as dimensions, contour and the like, utilizing a conductive capacitance probe having a high degree of sensitivity and resolution as well as proximity sensing capabilities, which is moved with respect to the workpiece without need for the probe to contact the workpiece. In addition, the probe movement; i.e., velocity and/or direction within a stand-off band, are at least in part, controlled by the capacitance being measured by the probe, thus relying on the probe's proximity sensing capabilities to control probe movement to provide a number of real time process controls to significantly speed any measurement procedure.
2. Summary of the Prior Art
Two and three dimensional co-ordinate measuring machines are well known in the art, which typically comprise a head supported for two or three-dimensional motion relative to a fixed structure. The movable head supports a stylus probe which is generally in the form of a straight rod with a small contact ball at the tip disposed away from the head. The machine includes a plurality of drive means for moving the head and, accordingly, the probe in two or three coordinate axes, and a monitoring means for instantly monitoring and recording the position of the probe with reference to the co-ordinate axes. Means is also provided for generating a signal when the probe comes into contact with the workpiece, which will stop the driving motion of the probe where its position is then recorded. Co-ordinate measurements of a workpiece mounted to the fixed structure are determined by moving the head in one or more directions and reading the co-ordinate position of the probe relative to a given datum when the probe contacts the workpiece. State of the art co-ordinate measuring machines are capable of monitoring and recording probe positions to an accuracy of 0.00025 mm (0.00001 inch).
Because the co-ordinate measuring machines of the prior art rely on physical contact to measure the co-ordinate positions of the workpiece surfaces, the operating sequence tends to be rather prolonged and time consuming. In addition, it is generally known that contact probes are expensive to manufacture and, due to the physical contact, are subject to wear and corrosion. It should be apparent that even a small amount of wear or corrosion can introduce significant errors to the machine's accuracy capabilities to 0.0005 mm.
In addition to the above, inherent difficulties and inaccuracies are built into the prior art system and techniques for determining a contact position which tend to detract from the accuracy capabilities of the co-ordinate measuring machine as whole. For example, setting the probe to an accurate zero starting position cannot be effected rapidly, as the probe must be brought into contact with the workpiece, and then carefully adjusted so that contact is maintained without any probe pressure against the workpiece, or any bending or deflection of the probe. More importantly, however, it is known that a mere light touch contact of the probe with the workpiece, without more, is not normally sufficient to activate the stop signal. Rather, the stop signal is effected only after the probe has been bent, deflected or in some way disturbed. While it would be desirable to stop the motion of the head at the exact instant of probe contact, the contact signal does not normally occur immediately upon contact, and the drive mechanism cannot be stopped instantly upon receipt of the stop signal. Rather, the head continues to move by a minute amount relative to the probe tip before the stop signal can be effected, with an even further minute movement before the stop signal effects a complete stop of the drive mechanism. As a result, the probe stylus is bent or deflected under the operating force necessary to generate and transmit the stop signal, as well as overcome the momentum of the drive means after the stop signal has been transmitted.
Since such probe bending or deflection is inherent, it is normally taken into account in the measuring process. The amount of relative movement between the head and the probe tip is referred to as the "bending allowance", and is deducted from the distance measured by the machine. To measure from a surface where such deflection has already occurred, a starting bending allowance must again be deducted from the distance measured, or else the probe re-set to a proper zero starting point. Accordingly, consecutive measurements of different surface dimensions can be a relatively prolonged procedure.
To be reasonably valid, any such "bending allowance" must first be based on probe movement at a uniform velocity, regardless of the dimension being measured, and secondly any bending force must be applied perpendicularly to the elongated orientation of the stylus probe. Therefore, while it would be desirable to move the probe at different velocities depending on the magnitude of the dimension being measured, the probe velocity must be fixed to the value used in establishing the bending allowance. Secondly, it should be apparent that any probe contact with a workpiece surface inclined at an angle to the direction of the probe movement will cause a lateral bending force which can deviate from the pre-set bending allowance. It should be further apparent that the application of any bending force vector which would tend to bend or deflect the probe towards the head, could cause the probe to be damaged.
It is generally known, however, that even at constant velocities and perpendicular bending forces, the forces necessary to effect a given bend or deflection, are not the same for different directions of application of the bending force. Therefore, application of a given bending allowance, regardless of the direction of the force, will inherently induce errors into the measurement determination.
While moving the probe at a relatively slow velocity will tend to minimize errors by minimizing probe bending or deflection, the inaccuracy is not completely eliminated but rather reduced in magnitude; and slowing of the fixed probe velocity will merely prolong the overall operation time, adversely affecting the equipment's efficiency.
To overcome the above problem, many different types of stylus mounting systems have been devised to permit stylus deflection without bending, which further include calibrating means for determining the extent of deflection through a variety of complex probe mounting systems. These mounting systems are not only intricate, complex and costly, but still do not achieve the degree of exacting measurements desired, and can often be damaged or knocked out of adjustment by the probe's impact with a workpiece surface.
There are a number of different types of contact probes which have been designed to overcome the bending and deflection problem such as radio frequency probes, analog touch probes, LVDT probes and even others. Radio frequency probes, for example, attenuate a radio frequency signal upon contact with a metallic workpiece. While these probes are capable of transmitting a stop signal immediately upon contact with the workpiece without any probe bending or deflection, the drive mechanism cannot be stopped immediately upon receipt of the stop signal, as momentum of the drive means must still be overcome. In addition, such arrangements are prone to electromagnetic noise interference with the radiated radio frequency signal which can lead to false indications of probe contact. Analog touch probes, sometimes referred to as "touch-fire" probes, rely specifically on the degree of pivoting of the probe upon contact with the workpiece to measure the workpiece surface and are capable of a direct contact tracing of a workpiece surface. To obtain reliable data, however, analog touch probes must be moved at a relatively slow speed which severely limits the number of measured reference points that can be achieved, and are, therefore, not practical for commercial applications. LVDT probes, like analog touch probes, rely on a constant deflection of the probe while the probe is moved very slowly along the surface of the workpiece. In addition to being very slow, these probes are subject to a significant degree of frictional wear.
It should further be apparent that any type of contact probe in motion, is "blind", and therefore, when moving towards a workpiece surface, has no way of sensing or "knowing" that it is approaching a surface or that contact is imminent. Therefore, the contact probe will make contact with the workpiece surface as the probe is moving at its fixed constant velocity. As noted above, contact at a fixed constant velocity is, nevertheless, essential so that a reasonably valid bending allowance can be subtracted from the measured distance of travel. Therefore, it is not only essential that the probe velocity be constant, but it must also be relatively modest so that meaningful bending allowances can be applied, and to further assure that the probe does not contact the workpiece with sufficient velocity to cause any damage to the probe, or any misalignment in the probe mounting structure and deflection measuring apparatus. While it would be advantageous with respect to real time controls to permit the probe to move at a higher velocity, particularly when traversing relatively long distances, it is apparent that anything more than a modest velocity cannot be utilized.
While prior art co-ordinate measuring apparatus and techniques can and have been applied to the measuring of workpiece surface contour to determine a profile line, it should be apparent that since probe contact and deflection are required to record a surface position, that the procedure whereby the probe will "follow" or "trace" the surface is rather cumbersome. To effect such a tracing movement, the probe must be programmed to move along first and second mutually perpendicular axes in a series of steps to alternately make and break contact with the workpiece, thus defining the profile by a series of contact points. The closer the contact points are, the more accurate the determined profile will be. If there is a considerable difference in the length of movement between the two axes, as a result of a very shallow or very steep workpiece surface incline, the contact points may become so spaced that resolution is seriously affected. It should be further apparent that the time necessary to perform a reasonably accurate complete profile can become exceptionally long. In addition, this technique is necessarily based upon probe contact which is not perpendicular to the direction of probe movement. Therefore, there may be inherent inaccuracies in the bending allowances applied.
As distinguished from contact probes discussed above, non-contacting capacitance probes have been utilized in prior art techniques for measuring conductive workpiece surface dimensions and characteristics. Such probes have established a highly useful role in industrial manufacturing and in the machine tool industry, in particular, for the characterization of surface properties. As the art is now aware, these sensors are based on the employment of a variable capacitance, due to the coupling of the sensor with a surface to be evaluated, by techniques such as controlling the rate of an oscillator circuit, so that the frequency of the oscillator is directly determined and altered as a function of surface characteristics.
Most of the prior art practices utilizing capacitance sensing probes are based on the use of probes that incorporate a plurality of capacitor sensors so that the differences in capacitance as measured by the various sensors is a function of the workpiece surface characteristic sought. For example, U.K. Patent No. 2,100,441, issued to Wolfendale, teaches a method of determining the contour of an unknown conductive surface or a diameter of a bore utilizing a non-contacting probe, which comprises a probe having a plurality of capacitor sensors positioned at or near the probe tip. According to one technique taught in the patent, a probe with a plurality of side-by-side sensors is held stationary at a given spaced relationship from the unknown surface; i.e., at a predetermined stand-off distance, and the capacitance values of the various capacitor elements are monitored. A variation in the capacitance readings indicates a variation in spacing between the sensors and the workpiece, thereby indication the workpiece surface contour. According to another technique taught in the patent, a general purpose sensor, having a plurality of sensor elements equally spaced abound the tip, is moved generally parallel to the workpiece surface while keeping the capacitance constant. The probe therefore "traces" the workpiece at a given "null point" or stand-off distance from the workpiece so that its path, less the stand-off distance, is an accurate measurement of the workpiece surface profile.
U.S. Pat. No. 4,816,744, issued to Papurt, et al., teaches a process and apparatus for measuring an inside dimension of a workpiece, such as a bore diameter, by positioning the probe at a given null point adjacent to one workpiece surface, and then moving the probe to an equal null point adjacent to the other workpiece surface, e.g. diametrically across a bore, and measuring the distance of probe movement with a laser interferometer. The measured distance or diameter is determined to be equal to the distance traveled by the probe plus two times the null point distance; i.e., the distance the probe center is spaced from the workpiece surface at the null point positions. While the probe utilized in this invention comprises a single capacitor sensor, and is therefore more amenable to general purpose applications, the technique is not universally applicable to general co-ordinate measuring techniques, and requires that the probe be started and stopped at the predetermined null point so that the start and stop stand-off positions are known. Additionally, starting and stopping the probe at a given null point is time consuming as the probe must be moved slowly and adjusted in incremental steps to achieve the exact null point reading, and therefore, does not provide any meaningful real time process controls.
All prior art workpiece measuring processes utilizing non-contacting capacitance probes rely on the null point technique regardless of the nature of the of parameters sought. In measuring distance or locations of workpiece surfaces, it has always been the practice that to avoid contact, the probe must be started, stopped or positioned at a null point with reference to the surface in interest as indicated by the correlating null point capacitance, so that the distance from the probe to the workpiece surface is know and can be considered in the final measurement determination. Even the prior art surface "tracing" techniques, as discussed above with reference to the Wolfendale patent, rely on such a null point capacitance to control the probe motion. Specifically, the probe is positioned and started at a starting null point where its displacement from the workpiece surface is known. Then the probe is moved generally parallel to the workpiece surface to be traced in a continuing series of incremental movements, and the change in capacitance at the end of each minute movement monitored. If the capacitance increases, the probe must be moved towards to workpiece until the null point capacitance is reached. If the capacitance increases, the probe is moved away from the workpiece surface until the null point capacitance is reached. In either situation, the probe position is recorded only after it has been adjusted to the exact null point. Even though the individual adjustments can done rather rapidly with analog or digital computations, it still requires a very large number of continuing adjustment or stepping of the probe position towards or away from the workpiece to locate the null point following each incremental lateral movement. It should be apparent, therefore, that this technique in not significantly different from the above described procedure of making and breaking contact with contact probes. Therefore, despite the use of analog or digital controls, the need to incrementally stop the probe movement to adjust it to the null point, is still quite time consuming.